Thorium-based fuel design for pressurized heavy water reactors

ABSTRACT

Thorium-based fuel bundles are used in existing PHWR reactors (e.g., Indian 220 MWe PHWR, Indian 540 MWe PHWR, Indian 700 MWe PHWR, CANDU 300/600/900) in place of conventional uranium-based fuel bundles, with little or no modifications to the reactor. The fuel composition of such bundles is 60+ wt % thorium, with the balance of fuel provided by low-enriched uranium (LEU), which has been enriched to 13-19.95%  235 U. According to various embodiments, the use of such thorium-based fuel bundles provides (1) 100% of the nominal power over the entire life cycle of the core, (2) high burnup, and (3) non-proliferative spent fuel bundles having a total isotopic uranium concentration of less than 12 wt %. Reprocessing of spent fuel bundles is also avoided.

CROSS REFERENCE

This application claims the benefit of priority from U.S. ProvisionalApplication No. 63/186,990, filed May 11, 2021, titled “Thorium-BasedFuel Design For Pressurized Heavy Water Reactors,” the entire contentsof which are hereby incorporated by reference herein.

BACKGROUND 1. Field of the Invention

Various embodiments relate generally to fuel pellets, pins, bundles, andcore loading for a pressurized heavy water nuclear reactor (“PHWR”), andspecifically relate to a thorium-based fuel composition for suchpellets, pins, bundles, and cores.

2. Description of Related Art

The 220 MWe PHWR is a heavy water moderated and heavy water cooledreactor with a reactor vessel 10. The reactor comprises 306 fuelchannels in the form of horizontal pressure tubes with coolant flow athigh pressure. Each channel contains 12 fuel bundles but only 10 arelocated in the active core. To isolate the hot fuel channels from thecolder moderator, the pressure tubes are loaded in calandria tubes thatare surrounded by heavy water moderator. The pressure tubes are arrangedin square lattice with a lattice pitch of 22.84 cm. The geometry of thereactor core 20 including the heavy water moderator reflector andexemplary fuel bundles are shown in FIG. 1. In FIG. 1, only twoexemplary bundles are shown, but it should be understood that additionalfuel bundles (not shown in FIG. 1) are provided in each of the 306channels.

The fuel bundle is a cluster of fuel pins with 19 pins per bundle. Theoverall length of the fuel bundle is 49.53 cm. The fuel is in the formof uranium dioxide pellets clad in zircaloy-4 pins. The fuel stacklength is 48.1 cm and the fuel pins are sealed by end-caps welded to theends of the fuel pin. To maintain the structural integrity of the fuelbundles, two end-plates are welded to the end-caps of the fuel pins.

In addition to the fuel channels, the reactor core comprises a number ofreactivity control devices. All reactivity devices are perpendicular tothe fuel channels and penetrate the reactor pressure vessel from top tobottom. Reactor power regulation is provided by the means of two powerregulating rods that are normally 80% inserted. For the control of thepower shape and for xenon override, 4 adjuster rods are installed whichare normally fully inserted. Reactor shutdown is provided by the meansof the primary shutdown system which comprises 14 shutdown rods normallyparked out-of-core. In case of emergencies and failure of the primaryshutdown system, the shutdown functionality can be providedindependently by the secondary shutdown system that comprises 12 liquidpoison compartments. The liquid compartments are normally filled byhelium gas, and when required, liquid neutron poisons are injected inshort time to provide safe reactor shutdown. In addition, the reactorhas two shim rods which are used to support shim mode operation whenrequired; these are normally parked out-of-core.

Canadian Patent Application Publication No. 2 810 133 discloses anuclear fuel bundle containing thorium and a nuclear reactor comprisingthe same. See also U.S. Patent Application Publication No. 2016/0035441.

SUMMARY

One or more non-limiting embodiments provide thorium-based fuel (e.g.,fuel pellet(s), fuel pin(s), and/or fuel bundle(s)) for a PHWR reactor,and a method of making and using the same, as disclosed and claimedherein.

One or more non-limiting embodiments provide a fresh fuel pelletconfigured to be used in a pressurized heavy water reactor. The fuelpellet includes thorium and uranium fuel. A fuel composition of the fuelpellet is between 55 and 90 wt % thorium, and between 10 and 45 wt %uranium. A ²³⁵U enrichment of the uranium is between 10.5% and 20%.

According to one or more of these embodiments, the pellet is annular inshape with a through-hole.

According to one or more of these embodiments, the through-hole has adiameter of between 0.3 and 1.0 cm.

According to one or more of these embodiments, the fuel composition isbetween 70 and 90 wt % thorium.

According to one or more of these embodiments, the ²³⁵U enrichment ofthe uranium is between 15% and 19%.

One or more non-limiting embodiments provides a fuel pin configured tobe used in a pressurized heavy water reactor. The fuel pin includes asealed tube, and a plurality of fuel pellets according to one or more ofthe embodiments disclosed herein. The plurality of fuel pellets aredisposed inside the sealed tube.

According to one or more of these embodiments, a fuel composition of thefuel pin is between 70 and 85 wt % thorium.

According to one or more of these embodiments, each of the plurality offuel pellets includes burnable poison.

One or more non-limiting embodiments provides a fuel bundle configuredto be used in a pressurized heavy water reactor. The fuel bundleincludes a plurality of fuel pins according to one of the embodimentsdisclosed herein. A fuel composition of at least one of the plurality offuel pins differs from a fuel composition in at least one other of theplurality of fuel pins.

According to one or more of these embodiments, the fuel bundle is shapedand configured to be used in a 220 MWe PHWR; the plurality of fuel pinshas exactly 19 fuel pins including one central fuel pin, sixintermediate fuel pins disposed radially outwardly from the one centralfuel pin, and twelve outer fuel pins disposed radially outwardly fromthe six intermediate fuel pins. A thorium weight percentage of the fuelcomposition of the central fuel pin is lower than a thorium weightpercentage of the fuel composition of each of the twelve outer fuelpins.

According to one or more of these embodiments, the fuel composition ofeach of the central fuel pin and six intermediate fuel pins has athorium content of between 55 and 75 wt %; the fuel composition of eachof the twelve outer fuel pins has a thorium content of between 65 and 90wt %; and each of the twelve outer fuel pins has a higher wt % thoriumcontent than each of the central and six intermediate fuel pins.

According to one or more of these embodiments, the fuel composition ofthe central fuel pin has a lower wt % thorium content than in each ofthe six intermediate fuel pins; and the fuel composition of each of thesix intermediate fuel pins has a lower wt % thorium content than in eachof the twelve outer fuel pins.

According to one or more of these embodiments, the fuel composition ofthe central fuel pin has a thorium content of between 55 and 70 wt %;the fuel composition of each of the six intermediate fuel pins has athorium content of between 60 and 80 wt %; and the fuel composition ofeach of the twelve outer fuel pins has a thorium content of between 65and 90 wt %.

According to one or more of these embodiments, the fuel bundle furtherincludes burnable poison disposed in the sealed tube of each of the sixintermediate fuel pins.

According to one or more of these embodiments, burnable poison is notdisposed in the sealed tube of any of the outer fuel pins.

According to one or more of these embodiments, the fuel bundle furtherincludes burnable poison disposed in the sealed tube of each of thecentral and six intermediate fuel pins, wherein burnable poison is notdisposed in the sealed tube of any of the outer fuel pins.

According to one or more of these embodiments, the burnable poisondisposed in the sealed tube of each of the central and six intermediatefuel pins includes Europium.

According to one or more of these embodiments, the fuel bundle includesburnable poison disposed in the sealed tube of each of the twelve outerfuel pins.

According to one or more of these embodiments, burnable poison is notdisposed in the sealed tube of the central fuel pin or any of theintermediate fuel pins.

According to one or more of these embodiments, burnable poison is notdisposed in the sealed tube of any of the fuel pins of the fuel bundle.

According to one or more of these embodiments, a ²³⁵U enrichment of theuranium of each of the central, intermediate, and outer pins is at least12%.

According to one or more of these embodiments, a ²³⁵U enrichment of theuranium of each of pellets of each of the central, intermediate, andouter pins is at least 15%.

According to one or more of these embodiments, a ²³⁵U enrichment of theuranium of each of the pellets of each of the twelve outer pins is lowerthan a ²³⁵U enrichment of the uranium of each of the pellets of each ofthe central and six intermediate pins.

According to one or more of these embodiments the fuel bundle is shapedand configured to be used in a CANDU PHWR; and the plurality of fuelpins include exactly 37 fuel pins including one central fuel pin, sixfirst ring fuel pins disposed radially outwardly from the one centralfuel pin, twelve second ring fuel pins disposed radially outwardly fromthe six first ring fuel pins, and eighteen outer fuel ring pins disposedradially outwardly from the twelve second ring fuel pins.

According to one or more of these embodiments, a thorium weightpercentage of the fuel composition of the central fuel pin is lower thana thorium weight percentage of the fuel composition of the second ringpins and of the outer ring pins.

According to one or more of these embodiments: a thorium weightpercentage of the fuel composition of the central fuel pin and of thefirst ring fuel pins is between 50 and 70 wt %; a thorium weightpercentage of the fuel composition of the second ring fuel pins isbetween 60 and 90 wt %; and a thorium weight percentage of the fuelcomposition of the outer ring fuel pins is between 75 and 99 wt %.

According to one or more of these embodiments, a ²³⁵U enrichment of theuranium of the central fuel pin and first ring fuel pins is higher thana ²³⁵U enrichment of the uranium of the second ring pins or outer ringpins.

According to one or more of these embodiments, a ²³⁵U enrichment of theuranium of the second ring fuel pins is higher than a ²³⁵U enrichment ofthe uranium of the outer ring pins.

According to one or more of these embodiments, the central pin, firstring pins, and second ring pins each include burnable poison.

According to one or more of these embodiments, the burnable poisonincludes Europium Oxide.

According to one or more of these embodiments, the outer ring pins donot include burnable poison.

One or more embodiments provides a pressurized heavy water reactorincluding a reactor vessel, and a core disposed in the reactor vessel.The core includes a plurality of fuel bundles according to one or moreof the embodiments disclosed herein. The plurality of fuel bundlesinclude a first type of fuel bundle and a second type of fuel bundle.The first type of fuel bundle includes burnable poison. The second typeof fuel bundle does not include burnable poison.

According to one or more of these embodiments, the first and secondtypes of fuel bundles are identical to each other except for theinclusion of burnable poison in the second type of fuel bundle.

One or more embodiments provides a fuel pellet configured to be used ina pressurized heavy water reactor. The fuel pellet includes thorium anduranium fuel. A fuel composition of the fuel pellet is between 55 and 90wt % thorium, and between 10 and 45 wt % uranium. A ²³⁵U enrichment ofthe uranium is between 5% and 20%. The pellet is annular in shape with athrough-hole.

According to one or more of these embodiments after removing the bundle,pins, and/or pellets from the reactor, a total isotopic uraniumconcentration within the spent fuel pellets, pins, and/or bundles (as aweight percentage of the spent fuel pellet(s), pin(s), and bundle), isless than 12, 11, 10, 9, 8, and/or 7%.

One or more of these and/or other aspects of various embodiments of thepresent invention, as well as the methods of operation and functions ofthe related elements of structure and the combination of parts andeconomies of manufacture, will become more apparent upon considerationof the following description and the appended claims with reference tothe accompanying drawings, all of which form a part of thisspecification, wherein like reference numerals designate correspondingparts in the various figures. In one embodiment, the structuralcomponents illustrated herein are drawn to scale. It is to be expresslyunderstood, however, that the drawings are for the purpose ofillustration and description only and are not intended as a definitionof the limits of the invention. In addition, it should be appreciatedthat structural features shown or described in any one embodiment hereincan be used in other embodiments as well. As used in the specificationand in the claims, the singular form of “a”, “an”, and “the” includeplural referents unless the context clearly dictates otherwise.

All closed-ended (e.g., between A and B) and open-ended (greater than C)ranges of values disclosed herein explicitly include all ranges thatfall within or nest within such ranges. For example, a disclosed rangeof 1-10 is understood as also disclosing, among other ranges, 2-10, 1-9,3-9, etc. Similarly, where multiple parameters (e.g., parameter C,parameter D) are separately disclosed as having ranges, the embodimentsdisclosed herein explicitly include embodiments that combine any valuewithin the disclosed range of one parameter (e.g., parameter C) with anyvalue within the disclosed range of any other parameter (e.g., parameterD).

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of various embodiments as well as otherobjects and further features thereof, reference is made to the followingdescription which is to be used in conjunction with the accompanyingdrawings, where:

FIG. 1 is a diagrammatic end view of a PHWR reactor according to one ormore embodiments;

FIG. 2 is a diagrammatic end view of a fuel bundle and a portion of thereactor of FIG. 1.

FIG. 3A is a perspective view of a fuel pellet of the fuel bundleillustrated in FIG. 2;

FIG. 3B is a side view of the fuel pellet in FIG. 3A;

FIG. 3C is a cross-sectional view of the fuel pellet of FIG. 3A, takenalong the line 3C-3C in FIG. 3B.

FIG. 4 is a table showing the composition of the pellets, pins, and fuelbundles in FIGS. 2-3, according to one or more non-limiting embodiments.

FIG. 5 is a table showing the composition of the pellets, pins, and fuelbundles in FIGS. 2-3, according to one or more non-limiting embodiments.

FIG. 6 is a diagrammatic end view of a fuel bundle and a portion of areactor according to one or more non-limiting alternative embodiments ofa CANDU reactor.

FIG. 7 is a table showing the composition of the pellets, pins, and fuelbundles, according to one or more non-limiting embodiments of thereactor illustrated in FIG. 6.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The 220 MWe PHWR is a standardised PHWR design developed and operated inIndia. This well known type of reactor will hereinafter be referred toas an “220 Mwe PHWR,” even if the reactor is not physically located inIndia.

The Indian 220 Mwe PHWR uses natural uranium (i.e., non-enricheduranium) in a low burnup open fuel cycle. Due to the use of naturaluranium and the potential of exploitation for plutonium production aswell as the concerns with nuclear waste produced, the current design isnot favoured for export and its utilisation is limited to domesticelectricity production. With the 220 Mwe PHWR, spent fuel has a lowburnup. Various non-limiting embodiments of the present inventionprovide an alternative fuel cycle for use in the 220 Mwe PHWR to addressthe proliferation and nuclear waste concerns. Various non-limitingembodiments of the present invention are based on the adaptation of the220 Mwe design for a high burnup once-through fuel cycle based onthorium, providing a 220 Mwe ThPHWR 5 with a reactor 10 housing a core200 that comprises thorium-LEU fuel bundles 100.

As shown in FIG. 2, the reactor 10 includes a plurality of calandriatubes 30 and pressure tubes 40 that define the channels into which thefuel bundles 100 are placed. The calandria tubes 30 and pressure tubes40 are well-known components of a conventional 220 Mwe PHWR to which oneor more embodiments are directed, so an exhaustive description of theseconventional components of a 220 Mwe PHWR are omitted.

According to various embodiments, a thorium based fuel cycle reduces oreliminates any proliferation risks given that fissile plutonium breedingis insignificant and uranium-233, because of associated uranium-232contamination, is not practical for use in nuclear weapons. On the otherhand, a high burnup fuel cycle is favoured as the amount of radioactivewaste generated is significantly reduced. According to variousnon-limiting embodiments, a thorium-LEU fuel in PHWR may provide thebest proliferation resistance among all water reactor types.

According to various non-limiting embodiments of the present invention,the overall systems and plant design of the 220 Mwe PHWR (see FIG. 1)are retained from the current 220 Mwe PHWR to form the 220 Mwe ThPHWR 5,10. According to various non-limiting embodiments of the presentinvention, the fuel bundle 100 delivers a high burnup thorium fuelcycle.

According to various non-limiting embodiments of the present invention,the impacts of the design changes (converting a 220 Mwe PHWR into a 220Mwe ThPHWR 5, 10) are limited to the core 20 characteristics rather thanthe overall plant 5, which reduces the effort required to implement andlicense the modified design.

According to various non-limiting embodiments of the present invention,the impacts of the design modification may include one or more of thefollowing:

-   -   i) The reactor physics characteristics of the core 20 of the 220        Mwe ThPHWR 5, 10 are completely different due to the changes in        the fuel composition and the unique neutronic characteristics of        thorium fuels.    -   ii) The adaptation of the high burnup cycle may pose challenges        on the structural integrity of the fuel bundles 100 given that        the structure of the current/conventional fuel bundles is used        in a low burnup fuel cycle.

According to various non-limiting embodiments of the present invention,a thorium fuel cycle is based on the following restrictions:

-   -   i) Any design changes to the fuel bundle are limited to the        internal dimensions of the fuel pins with the overall geometry        and structure of the fuel bundle retained from the        current/conventional design. With this restriction, the majority        of the safety and design analysis of the reactor core will be        applicable to various non-limiting embodiments of the present        invention.    -   ii) In optimising the design of the fuel bundle and reactor        core, the current power limits imposed on the operating 220 Mwe        PHWR are to be respected for the 220 Mwe ThPHWR 10. This        restriction implies that the current design of the heat transfer        system and the thermal-hydraulics apply to various non-limiting        embodiments of the present invention, without a need for any        modifications.

According to various non-limiting embodiments of the present invention,a thorium fueled once-through high burnup nuclear reactor cycle isprovided.

One or more non-limiting embodiments retains, to the extent possible,all design features of the 220 Mwe PHWR. Such an approach may lead tominimising the effort required to establish the safety case of thereactor and allows implementation of one or more non-limitingembodiments in the near future. On the reactor core side, the geometryof the core, the number of fuel channels and the size of the core arenot changed. On the fuel bundle side, the external dimensions of fuelpins and the shape of the fuel bundle are not changed. That is, one ormore non-limiting embodiments provide a fuel bundle 100 that contains 19fuel pins with overall length of 49.53 cm. The changes in the fuelbundle 100 design according to various non-limiting embodiments may belimited to:

-   -   i) The fuel composition; and/or    -   ii) The internal design and structure of the fuel pin 200.

As explained in greater detail below, a ThPHWR 5 includes a reactor andreactor vessel 10 (shown as a circle in FIG. 1) that houses a reactorcore 20 that comprises fuel bundles 100. The fuel bundles 100 comprisefuel pins 200. The fuel pins 200 comprise sealed tubes 210 that arefilled with fuel pellets 300.

The fuel composition of each pellet 300 according to one or moreembodiments is a thorium-uranium mixed oxide with low enrichment uranium(LEU). In the initial design studies, the fuel composition was specifiedto 80% by weight thorium with 20% uranium enrichment.

As used herein, the terms “thorium” and “thorium oxide” are usedinterchangeably, and both refer to thorium oxide (ThO₂). Similarly, asused herein, the terms “uranium” and “uranium oxide” are usedinterchangeably, and both refer to uranium oxide (UO₂).

Fuel performance analyses show that large fission gas release and fuelpin internal pressure are expected under high burnup conditions if thecurrent design was to be used. In addition, excessive cladding damagewas predicted. To address these two issues, the fuel bundle 100 designwas modified as follows:

-   -   i) To accommodate the fission gases and reduce the internal pin        200 pressure, a central hole 310 was introduced into the fuel        pellets 300 (see FIGS. 2, 3).    -   ii) To reduce cladding-pellet interactions, the size of the        initial fuel-cladding gap was increased.    -   iii) To reduce the impacts of irradiation damage on the fuel pin        integrity, the thickness of the cladding was increased.    -   iv) Finally, advanced cladding material (e.g., zirconium,        Zirlo™) with proven performance at high burnups was selected.

Fuel Pellets 300

As shown in FIG. 3, each fuel pellet 300 has an annular shape that isgenerally cylindrical with a central axially-extending through-hole 310.According to various non-limiting embodiments, the fuel pellets 300 havethe following physical parameters:

-   -   i) Fuel pellet 300 outer diameter: at least 0.9, 1.0, 1.1, 1.2,        and/or 1.3 cm, less than or equal to than 2.0, 1.9, 1.8, 1.7,        1.6, 1.5, 1.4, 1.3, 1.2, 1.1, and/or 1.0 cm, between any two        such values (e.g., between 0.9 and 2.0 cm, between 1.1 and 1.7        cm, between 1.2 and 1.4 cm, between 1.3 and 1.4 cm, about 1.376        cm);    -   ii) Fuel pellet center hole diameter: at least 0.1, 0.2, 0.3,        0.4, 0.5, 0.6, 0.7, 0.8, and/or 0.9 cm, less than or equal to        1.1, 1.0, 0.9, 0.8, 0.7, 0.6, 0.5, 0.4, and/or 0.3 cm, and/or        between any two such values (e.g., between 0.1 and 1.1 cm,        between 0.2 and 1.0 cm, between 0.4 and 0.8 cm, between 0.5 and        0.7 cm, about 0.6 cm);    -   iii) Fuel pellet axial length: at least 0.6, 0.7, 0.8, 0.9, 1.0,        1.1, 1.2, 1.3, 1.4, and/or 1.5 cm, less than or equal to 2.0,        1.9, 1.8, 1.7, 1.6, 1.5, 1.4, 1.3, 1.2, 1.1, 1.0, 0.9, 0.8,        and/or 0.7 cm, and/or between any two such values (e.g., between        0.6 and 2.0 cm, between 0.8 and 1.2 cm, between 0.9 and 1.1 cm,        about 1.0 cm);    -   iv) Fuel pellet 300 true density 97%;    -   v) Fuel pellet surface roughness Similar to commercial PHWR        fuel—0.76 μm; and    -   vi) Fuel pellet grain size 10-60 μm.

As shown in FIG. 3, the intersection between the hole and the axial endsof the pellet 300 is chamfered, which may reduce pellet chipping duringloading and handling, and may reduce strains on the pellet and/or tubingwhen the pellet swells as a result of irradiation.

According to various embodiments, the fuel composition (on a weightpercentage basis) of each pellet 300 (and, by extension, each fuel pin200 made up of such pellets 300) comprises:

-   -   i) at least 50, 55, 60, 65, 70, 75, 80, 85, and/or 90 wt %        thorium (e.g., thorium oxide), less than or equal to 100, 95,        90, 85, 80, 75, 70, 65, 60, and/or 55% thorium, and/or between        any two such values (e.g., between 50 and 95 wt % thorium,        between 60 and 90 wt % thorium, between 65 and 85 wt % thorium);    -   ii) at least 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18,        19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34,        35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49,        and/or 50 wt % uranium oxide LEU, less than or equal to 50, 49,        48, 47, 46, 45, 44, 43, 42, 41, 40, 39, 38, 37, 36, 35, 34, 33,        32, 31, 30, 29, 28, 27, 26, 25, 24, 23, 22, 21, 20, 19, 18, 17,        16, 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, and/or 5 wt % uranium        oxide LEU, and/or between any two such values (e.g., between 5        and 50 wt % uranium oxide LEU, between 10 and 40 wt % uranium        oxide LEU, between 10 and 30 wt % uranium oxide LEU, about 30 wt        % uranium oxide LEU, about 15 wt % uranium oxide LEU); and    -   iii) optionally burnable poison such as boron oxide at a        concentration of at least 0.0, 0.05, 0.1, 0.15, 0.2, and/or 0.3        wt % (as a % of the overall pellet weight) burnable poison        (e.g., boron oxide), less than 1.0, 0.9, 0.8, 0.7, 0.6, 0.5,        0.4, 0.3, and/or 0.2 pellet wt %, and/or between any two such        values (e.g., between 0.0 and 1.0 pellet wt %, between 0.5 and        0.3 pellet wt %, between 0.1 and 0.3 pellet wt %, between 0.15        and 0.3 pellet wt %, between 0.2 and 0.3 pellet wt %, about 0.22        pellet wt %). If burnable poison other than boron oxide is used,        an equivalent amount/concentration of such other material may be        used. As explained below, such burnable poison may be included        in certain fuel pellets 300 a′ that are used in certain fuel        pins (e.g., the below-discussed middle ring fuel pins 200 b).        Unless otherwise specifically stated, all composition        percentages provided herein are weight percentages (wt %).

As explained in further detail below, some fuel pellets 300 omit uraniumentirely, such that the fuel composition of the resulting pellet 300 cis 100% thorium. As explained below the pellets 300 c are used inthorium fuel pins 200 d that are used in thorium bundles 100 c.

According to various embodiments, fuel pellets 300 of differing fuelcomposition may be used within a single pin 200 or different pins 200 a,200 b, 200 c, 200 d. For example, as shown in FIG. 4, according to oneor more embodiments:

-   -   i) the fuel composition of fuel pellets 300 a used in the        central fuel pins 200 a comprises 70% thorium and 30% uranium,    -   ii) the fuel composition of fuel pellets 300 a′ used in        intermediate ring fuel pins 200 b comprises about 70% thorium,        about 30% uranium, and 0.22 wt % boron oxide (or other burnable        poison),    -   iii) the fuel composition of fuel pellets 300 b used in certain        outer fuel pins 200 c comprises 85% thorium and 15% uranium, and    -   iv) the fuel composition of fuel pellets 300 c used in certain        other fuel pins 200 d and bundles 300 c comprises 100% thorium.

According to various embodiments, the LEU of the fuel pellets 300 has a²³⁵U enrichment of at least 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15,and/or 16%, a ²³⁵U enrichment of less than or equal to 20, 19, 18, 17,16, 15, 14, 13, 12, 11, 10, 9, 8, 7, and/or 6%, and or any value betweenany two such values (e.g., a ²³⁵U enrichment of between 5 and 20%,between 10 and 18%, between 12 and 16%, about 13%, about 15%). Accordingto various embodiments, the ²³⁵U enrichment of the LEU differs betweendifferent fuel pellets 300 within a single pin 200. According to variousembodiments, as shown in FIG. 4, the ²³⁵U enrichment of the LEU of thefuel pellets 300 a, 300 a′ used in one or more pins 200 a, 200 b (e.g.,13.0% enrichment) is lower than the enrichment of the LEU of the fuelpellets 300 b used in one or more other pins 200 c (e.g., 15%enrichment). In the embodiment illustrated in FIG. 4, the difference inenrichment is 2.0 absolute % (13% v. 15%). However, according toalternative embodiments, the absolute difference in enrichmentpercentage may be higher or lower: e.g.: at least 0.2, 0.3, 0.4, 0.5,0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9,2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0, 7.0, 8.0, 9.0, and/or 10.0absolute % difference, less than 10.0, 9.0, 8.0, 7.0, 6.0, 5.5, 5.0,4.5, 4.0, 3.5, 3.0, 2.5, 2.0, 1.9, 1.8, 1.7, 1.6, 1.5, 1.4, 1.3, 1.2,1.1, 1.0, 0.9, 0.8, 0.7, 0.6, 0.5, 0.4, 0.3, and/or 0.2 absolute %difference, and or between any two such values (e.g., a difference ofbetween 0.2 and 10.0 absolute %, a difference of 1.0-8.0 absolute %,about 2 absolute %). As used herein, the difference in enrichment meansabsolute, rather than relative difference in percentage, such that thedifference between 5% enriched LEU and 15% enriched LEU is 10% (15%minus 5%), not 200%.

Fuel Pins 200

As shown in FIG. 2, each fuel pin 200 comprises a sealed cladding tube210. According to various embodiments, the sealed tube 210 comprises anannular tube with end caps attached (e.g., via welding) to each end.Forty to fifty (e.g., 48) axially aligned fuel pellets 300 are disposedwithin the sealed tube 210, though greater or fewer pellets 300 could beused per pin 200 without deviating from the scope of the presentinvention. For example, fewer pellets 300 could be used to provide morespace for fission gas build up. More pellets 300 could be used, forexample if the pellets 300 are axially shortened. According to variousembodiments, the tube 210 has a 1.40 cm inner diameter and a 0.06 cmthickness. The tube 210, including its end caps, may comprise anysuitable material (e.g., zirconium alloy). According to variousembodiments, the interior cavity of the sealed tube 210 is filledpressurized via filler (e.g., helium) to 0.5 Mpa.

As illustrated in FIG. 4, different variations of the pin 200 are filledwith different types of fuel pellets 300 as follows:

-   -   i) Fuel pins 200 a comprise fuel pellets 300 a such that the        fuel composition of the pins 200 a matches that of the pellets        300 a (e.g., 70% thorium and 30% uranium (13% enriched));    -   ii) Fuel pins 200 b.

Fuel Bundles 100

As shown in FIG. 2, each fuel bundle 100 comprises a plurality of fuelpins 200. For the 220 Mwe ThPHWR 10 according to one embodiment, thefuel bundle 100 includes 19 fuel pins 200: one central fuel pin 200 a,six intermediate fuel pins 200 b disposed in a ring radially outwardlyfrom the central fuel pin 200 a, and twelve outer fuel pins 200 carranged in a ring radially outwardly from the intermediate ring of pins200 b.

As with conventional fuel bundles, circular end plates with openings forcoolant to pass there through are welded or otherwise attached to theaxial ends of the fuel pins 200 a, 200 b, 200 c to maintain the pins 200in the locations shown in FIG. 2.

With these considerations, as shown in FIG. 4, the fuel composition offuel bundles 100 a according to one or more embodiments was determinedas follows:

-   -   i) The central fuel pin 200 a contains fuel having a composition        of 70 wt % thorium and 30 wt % uranium (specifically 13% ²³⁵U        enrichment uranium). This fuel composition is achieved according        to various embodiments by filling the fuel pin 200 a with fuel        pellets 300 a.    -   ii) The fuel pins 200 b in the middle ring are generally similar        to the fuel pin 200 a, except that the fuel pins 200 b are        filled with fuel pellets 300 a′ that also include burnable        poison (e.g., 0.22 pellet wt % boron oxide or other equivalent        material and disposition combinations), rather than fuel pellets        300 a that omit burnable poison. The fuel pins 200 b and fuel        pellets 300 a′ therefore have slightly less thorium and uranium        than the fuel pins 200 a and fuel pellets 300 a because the        burnable poison displaces a small amount of thorium and uranium.    -   iii) The fuel composition of the fuel pins 200 c in the outer        ring is 85 wt % thorium and 15 wt % uranium (specifically 15%        ²³⁵U enrichment uranium). This fuel composition is achieved        according to various embodiments by filling the pin 200 c with        fuel pellets 300 b having a matching fuel composition.

The fuel composition of the fuel bundle 100 is summarised in FIG. 4.

According to various alternative embodiments, the fuel composition ofany of the fuel pins 200, 200 a, 200 b may be varied by using fuelpellets 300 having any of the above-discussed fuel compositions (orfurther alternative fuel compositions) without deviating from the scopeof the present invention.

As a result of these various combinations:

-   -   i) Th/U fuel bundles 100 a comprise:        -   (1) a Th/U central fuel pin 200 a comprising fuel pellets            300 a (70% Th; 30% U (enriched to 13%)) and no burnable            poison;        -   (2) six Th/U intermediate fuel pins 200 b comprising fuel            pellets 300 a′ (fuel composition of 70 wt % Th and 30 wt % U            (enriched to 13%); 0.22 pellet wt % boron oxide); and        -   (3) twelve outer fuel pins 200 c comprising fuel pellets 300            b (85 wt % Th; 15 wt % U (enriched 15%)) and no burnable            poison;    -   ii) Th/U fuel bundles 100 b may be generally identical to Th/U        fuel bundles 100 a, except that burnable poison is not included        in the pellets 300 a′ of the six intermediate fuel pins 200 b        (thus making these intermediate fuel pins 200 b substantively        identical to the central fuel pins 200 a), which results in a        fuel bundle 100 b that has no burnable poison and comprises:        -   (1) a Th/U central fuel pin 200 a comprising fuel pellets            300 a (70% Th; 30% U (enriched to 13%)) and no burnable            poison;        -   (2) six Th/U intermediate fuel pins 200 b comprising fuel            pellets 300 a (70 wt % Th; 30 wt % U (enriched to 13%)) and            no burnable poison; and        -   (3) twelve outer fuel pins 200 c comprising fuel pellets 300            b (85 wt % Th; 15 wt % U (enriched 15%)) and no burnable            poison; and    -   iii) Th fuel bundles 100 c comprise fuel pins 200 d that contain        Th fuel pellets 300 c (100% thorium) and no burnable poison.

Alternative Compositions For Fuel Bundles, Pins, and Pellets

The composition of the fuel bundles 100 a, 100 b, pins 200 a, 200 b, 200c, and pellets 300 a, 300 a′, 300 b described herein may be modified ina variety of ways according to a variety of alternative embodiments.

According to one or more alternative embodiments of the fuel bundles 100a, 100 b, and/or fuel pins 200 a, 200 b, 200 c, the fuel pellets 300 a,300 a′, 300 b may utilize more highly enriched uranium than in one ormore of the above described embodiments (e.g., (a) at least 15, 16, 17,and/or 18% ²³⁵U enrichment, (b) less than or equal to 19.95, 19, and/or18% ²³⁵U enrichment, and/or (c) between any two such upper and lowervalues (e.g., between 15 and 19.95% ²³⁵U enrichment, between 16 and 19%²³⁵U enrichment, about 17.8% ²³⁵U enrichment).

According to one or more of these embodiments, the ²³⁵U enrichment levelof the uranium in each of the pellets 300 a, 300 a′, 300 b may bestandardized across many or all of the fuel bundles 100 a, 100 b, fuelpins 200 a, 200 b, 200 c, and/or fuel pellets 300 a, 300 a′, 300 b.

According to one or more of these embodiments, burnable poison may beprovided in the outer ring pins 200 c and pellets 300 b, rather than inthe middle ring fuel pins 200 b and pellets 300 a′. According to variousembodiments, the burnable poison used in the outer ring pins 200 c andpellets 300 b comprises Erbium Oxide at a concentration of (a) at least0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, and/or 1.3wt % of the pellet, (b) less than or equal to 3.0, 2.5, 2.4, 2.3, 2.2,2.1, 2.0, 1.9, 1.8, 1.7, 1.6, 1.5, 1.4, and/or 1.3 wt % of the pellet,and/or (c) between any two such values (e.g., between 0.1 and 3.0 wt %of the pellet, between 1.0 and 2.0 wt % of the pellet, about 1.3 wt % ofthe pellet).

According to one or more of these embodiments, the thorium concentrationof the fuel in the pellets 300 a, 300 a′ of the middle ring pins 200 bmay be higher than in one or more of the above-discussed embodiments,for example, (a) at least 71, 72, 73, 74, and/or 75 wt % of fuel, (b)less than or equal to 84, 83, 82, 81, 80, 79, 78, 77, 76, and/or 75 wt %of fuel, and/or (c) between any two such values (e.g., between 71 and 84wt % of fuel, between 72 and 78 wt % of fuel, about 75 wt % of fuel).

FIG. 5 illustrates the composition of the fuel bundles 100 a, 100 baccording to one or more of these embodiments.

According to various alternative embodiments, Th/U fuel bundles 100 maycomprise:

-   -   i) a Th/U central fuel pin 200 a comprising:        -   (1) fuel pellets 300 a with a fuel composition comprising            60% Th and 40% U (enriched to 15%), and        -   (2) 0.6 wt % Europium (Eu) burnable poison;    -   ii) six Th/U intermediate fuel pins 200 b comprising:        -   (1) fuel pellets 300 a′ with a fuel composition of 65 wt %            Th and 35 wt % U (enriched to 15%), and        -   (2) 0.6 wt % Europium (Eu) burnable poison; and    -   iii) twelve outer fuel pins 200 c comprising:        -   (1) fuel pellets 300 b with a fuel composition comprising 70            wt % Th and 30 wt % U (enriched 10%), and        -   (2) no burnable poison.            According to various non-limiting embodiments, the same type            of fuel bundles (e.g., as described immediately above) may            be used at all locations within the reactor in place of the            above-discussed bundles 100 a, 100 b, and 100 c, which had            divergent compositions.

According to various alternative embodiments, Th/U fuel bundles 100 maycomprise:

-   -   i) a Th/U central fuel pin 200 a comprising:        -   (1) fuel pellets 300 a with a fuel composition comprising            60% Th and 40% U (enriched to 16%), and        -   (2) 0.6 wt % Europium (Eu) burnable poison;    -   ii) six Th/U intermediate fuel pins 200 b comprising:        -   (1) fuel pellets 300 a′ with a fuel composition of 65 wt %            Th and 35 wt % U (enriched to 16%), and        -   (2) 0.6 wt % Europium (Eu) burnable poison; and    -   iii) twelve outer fuel pins 200 c comprising:        -   (1) fuel pellets 300 b with a fuel composition comprising 80            wt % Th and 20 wt % U (enriched 15%), and        -   (2) no burnable poison.            According to various non-limiting embodiments, the same type            of fuel bundles (e.g., as described immediately above) may            be used at all locations within the reactor in place of the            above-discussed bundles 100 a, 100 b, and 100 c, which had            divergent compositions.

CANDU 600 MWe Reactor Embodiments

FIG. 6 illustrates a reactor 1005 according to one or more non-limitingalternative embodiments. The illustrated reactor 1005 is generallysimilar to the above-discussed embodiments, so a redundant explanationof similar or identical structures and features is omitted. As shown inFIG. 6, the reactor vessel 1010 of the reactor 1005 houses a core 1020that comprises a plurality of calandria tubes 1030 and pressure tubes1040 that define the channels into which fuel bundles 1100 are placed.

In the illustrated embodiment, the reactor 1005, reactor vessel/housing1010, calandria tubes 1030, and pressure tubes 1040 are well-knowncomponents of a CANDU type reactor to which one or more non-limitingembodiments are directed, so an exhaustive description of theseconventional components of a CANDU reactor are omitted. Similarly, thedimensions and configurations of the fuel bundles 1100 and pins 1200 a,1200 b, 1200 c, 1200 d are likewise well-known in connection with theCANDU reactor to which various non-limiting embodiments are directed, soan exhaustive description of such dimensions and configurations isomitted.

As shown in FIG. 6, each fuel bundle 1100 comprises 37 fuel pins,including: one central fuel pin 1200 a containing fuel pellets 1300 a,six first ring fuel pins 1200 b containing fuel pellets 1300 a, twelvesecond ring fuel pins 1200 c containing fuel pellets 1300 b, andeighteen outer ring fuel pins 1200 d containing fuel pellets 1300 c.

As shown in the non-limiting embodiment illustrated in FIG. 7, thecomposition of the fuel pellets 1300 a, 1300 b, 1300 c of the fuel pins1200 a, 1200 b, 1200 c, 1200 d of the fuel bundles 1100 according to oneor more embodiments differ from those of a conventional 37-pin CANDUfuel bundle.

According to one or more embodiments of the fuel bundles 1100 and/orfuel pins 1200 a, 1200 b, 1200 c, and/or 1200 d, the fuel pellets 1300a, 1300 b, and/or 1300 c utilize uranium enriched to (a) at least 13,14, 15, 16, 17, and/or 18% ²³⁵U enrichment, (b) less than or equal to19.95, 19, 18, and/or 17% ²³⁵U enrichment, and/or (c) between any twosuch upper and lower values (e.g., between 13 and 19.95% ²³⁵Uenrichment, between 16 and 19% ²³⁵U enrichment, about 16, 17, 18, or 19%²³⁵U enrichment).

According to various non-limiting embodiments, the ²³⁵U enrichment ofthe fuel pellets 1300 a in the central fuel pin 1200 a and/or first ringfuel pins 1200 b are equal to each other. However, according to one ormore alternative embodiments, the enrichment level of the fuel pelletsin the central pin 1200 a may be higher or lower than the enrichment ofthe fuel pellets in the first ring fuel pins 1200 b.

According to various non-limiting embodiments, the ²³⁵U enrichment ofthe fuel pellets 1300 a in the central fuel pin 1200 a and/or first ringfuel pins 1200 b is higher than the ²³⁵U enrichment of the fuel pellets1300 b in the second ring pins 1200 c and/or the fuel pellets 1300 c inthe outer ring pins 1200 d by (a) at least 0.1, 0.2, 0.3, 0.4, 0.5, 0.6,0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0,2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, 3.0, 3.1, 3.2, 3.3, 3.4,3.5, 3.6, 3.7, 3.8, 3.9, 4.0, 4.5, 5.0, 5.5, 6.0, 6.5, 7.0, 7.5, and/or8.0 absolute %, (b) less than or equal to 8.0, 7.5, 7.0, 6.5, 6.0, 5.5,5.0, 4.5, 4.0, 3.9, 3.8, 3.7, 3.6, 3.5, 3.4, 3.3, 3.2, 3.1, 3.0, 2.9,2.8, 2.7, 2.6, 2.5, 2.4, 2.3, 2.2, 2.1, 2.0, 1.9, 1.8, 1.7, 1.6, 1.5,1.4, 1.3, 1.2, 1.1, 1.0, 0.9, 0.8, 0.7, 0.6, 0.5, 0.4, 0.3, and/or 0.2absolute %, and/or (c) any value between such upper and lower limits(e.g., between 0.1 and 8.0 absolute %, between 0.5 and 1.5 absolute %,between 1.5 and 2.5 absolute %, about 1 absolute %, about 3 absolute %).As used herein, ‘absolute %” of enrichment means percentage ofenrichment, not a percentage deviation from another enrichment value. Asa result, a fuel pellet 1300 a with a 19% enrichment has 2 absolute %higher enrichment than a pellet 1300 b with a 17% enrichment.

According to various non-limiting embodiments, the ²³⁵U enrichment ofthe fuel pellets 1300 b in the second ring pins 1200 c is higher thanthe ²³⁵U enrichment of the fuel pellets 1300 c in the outer ring pins1200 d by (a) at least 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0,1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4,2.5, 2.6, 2.7, 2.8, 2.9, 3.0, 3.1, 3.2, 3.3, 3.4, 3.5, 3.6, 3.7, 3.8,3.9, 4.0, 4.5, 5.0, 5.5, 6.0, 6.5, and/or 7.0 absolute %, (b) less thanor equal to 7.0, 6.5, 6.0, 5.5, 5.0, 4.5, 4.0, 3.9, 3.8, 3.7, 3.6, 3.5,3.4, 3.3, 3.2, 3.1, 3.0, 2.9, 2.8, 2.7, 2.6, 2.5, 2.4, 2.3, 2.2, 2.1,2.0, 1.9, 1.8, 1.7, 1.6, 1.5, 1.4, 1.3, 1.2, 1.1, 1.0, 0.9, 0.8, 0.7,0.6, 0.5, 0.4, 0.3, and/or 0.2 absolute %, and/or (c) any value betweensuch upper and lower limits (e.g., between 0.1 and 7.0 absolute %,between 0.5 and 1.5 absolute %, about 1 absolute %).

According to one or more non-limiting embodiments, the fuel pellets 1300a and/or 1300 b and/or the pins 1200 a, 1200 b, and/or 1200 c includeburnable poison. According to various embodiments, the burnable poisonused in the pellets 1300 a and/or 1300 b and/or pins 1200 a, 1200 b,and/or 1200 c comprises Europium Oxide at a concentration of (a) atleast 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, 0.11, 0.12, 0.13, 0.14, 0.15,0.16, 0.17, 0.18, 0.19, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0,1.1, 1.2, and/or 1.3 wt % of the pellet, (b) less than or equal to 3.0,2.5, 2.4, 2.3, 2.2, 2.1, 2.0, 1.9, 1.8, 1.7, 1.6, 1.5, 1.4, 1.3, 1.2,1.1, 1.0, 0.9, 0.8, 0.7, 0.6, 0.5, 0.4, 0.3, 0.2, 0.15, 0.14, 0.13,0.12, 0.11, 0.1, 0.09, 0.08, 0.07, 0.06, and/or 0.05 wt % of the pellet,and/or (c) between any two such values (e.g., between 0.05 and 3.0 wt %of the pellet, between 1.0 and 2.0% of the pellet, about 1.2 wt % of thepellet, between 0.05 wt % and 0.2 wt %, between 0.05 and 0.15 wt %).According to various embodiments, a burnable poison concentration in thefuel pellets 1300 a and/or central and first pins 1200 a, 1200 b ishigher than the burnable poison (e.g., Europium Oxide) concentration inthe fuel pellets 1300 b and/or second ring pins 1200 c by (a) at least0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, and/or 1.0 absolute wt%, (b) less than 2.0, 1.9, 1.8, 1.7, 1.6, 1.5, 1.4, 1.3, 1.2, 1.1, 1.0,0.9, 0.8, 0.7, 0.6, 0.5, 0.4, 0.3, 0.2, and/or 0.1 absolute wt %, and/or(c) between any two such values (e.g., between 0.1 and 2.0 absolute wt%, between 0.5 and 1.7 absolute wt %, about 1.1 absolute wt %).According to one or more non-limiting embodiments, the fuel pellets 1300a have a Europium Oxide concentration of 1.2 wt %, and the fuel pellets1300 b have a Europium Oxide concentration of 0.1 wt % such that thepellets 1300 a have a 1.1 absolute wt % higher Europium Oxideconcentration than the pellets 1300 b. According to one or morealternative embodiments, any other burnable poison (e.g., Erbium Oxide)may additionally and/or alternatively be used.

According to various alternative embodiments, burnable poison (e.g.,Europium Oxide and/or Erbium Oxide in the concentrations discussed inany of the above embodiments) may additionally and/or alternatively beprovided in the fuel pellets 1300 c and/or pins 1200 d.

According to one or more embodiments, the thorium fuel concentration(i.e., a wt % thorium as a portion of the overall fuel weight) in thefuel pellets 1300 a of the central pin 1200 a and/or first ring pins1200 b that is (a) at least 45, 50, 55, 57.5, 60, 62.5, and/or 65 wt %,(b) less than or equal to 90, 85, 80, 75, 70, 65, 62.5, and/or 60 wt %,and/or (c) between any two such values (e.g., between 45 and 90 wt %,between 50 and 70 wt %, between 55 and 65 wt %, between 57.5 and 62.5 wt%, about 65 wt %). According to various non-limiting embodiments, abalance of the pellet's fuel composition is uranium (e.g., 65 wt %thorium and 35 wt % uranium).

According to one or more embodiments, the thorium fuel concentration(i.e., a wt % thorium as a portion of the overall fuel weight) in thefuel pellets 1300 b of the second ring pins 1200 c is (a) at least 55,60, 65, 70, 72.5, 75, and/or 77.5 wt %, (b) less than or equal to 95,90, 85, 80, 77.5, 75, and/or 72.5 wt %, and/or (c) between any two suchvalues (e.g., between 55 and 95 wt %, between 65 and 85 wt %, between 70and 80 wt %, between 72.5 and 77.5 wt %, about 75 wt %). According tovarious non-limiting embodiments, a balance of the pellet's fuelcomposition is uranium (e.g., 75 wt % thorium and 25 wt % uranium).

According to one or more embodiments, the thorium fuel concentration(i.e., a wt % thorium as a portion of the overall fuel weight) in thefuel pellets 1300 c of the outer ring pins 1200 d is (a) at least 55,60, 65, 70, 75, 80, 82.5, 85, and/or 87.5 wt %, (b) less than or equalto 99, 95, 90, 87.5 85, and/or 82.5 wt %, and/or (c) between any twosuch values (e.g., between 55 and 99 wt %, between 75 and 95 wt %,between 80 and 90 wt %, between 82.5 and 87.5 wt %, about 85 wt %).According to various non-limiting embodiments, a balance of the pellet'sfuel composition is uranium (e.g., 85 wt % thorium and 15 wt % uranium).

According to one or more of these embodiments, the thorium fuelconcentration in the pellets 1300 c of the outer ring pins 1200 d ishigher than the thorium fuel concentration in the fuel pellets 1300 band/or 1300 c of the second ring pins 1200 c, the first ring pins 1200b, and/or the central pin 1200 a. According to various non-limitingexamples, the thorium fuel concentration (i.e., a wt % thorium as aportion of the overall fuel weight) in the pellets 1300 c is higher thanthe thorium fuel concentration in the pellets 1300 b by (a) at least 1,2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, and/or 15 absolute wt %, (b)less than 25, 20, 19, 18, 17, 16, 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5,4, 3, 2, and/or 1 absolute wt %, and/or (c) a value between any two suchvalues (e.g., between 1 and 25 absolute wt %, between 5 and 15 wt %,about 10 wt %). According to various non-limiting examples, the thoriumfuel concentration in the pellets 1300 c is higher than the thorium fuelconcentration in the pellets 1300 a by (a) at least 1, 2, 3, 4, 5, 6, 7,8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25,26, 27, 28, 29, and/or 30 absolute wt %, (b) less than 30, 29, 28, 27,26, 25, 24, 23, 22, 21, 20, 19, 18, 17, 16, 15, 14, 13, 12, 11, 10, 9,8, 7, 6, 5, 4, 3, 2, and/or 1 absolute wt %, and/or (c) a value betweenany two such values (e.g., between 1 and 30 absolute wt %, between 20and 30 wt %, about 25 wt %).

According to one or more embodiments, the thorium fuel concentration inthe pellets 1300 b of the second ring pins 1200 c is higher than thethorium fuel concentration in the fuel pellets 1300 a of the first ringpins 1200 b and/or central pin 1200 a. According to various non-limitingexamples, the thorium fuel concentration in the pellets 1300 b is higherthan the thorium fuel concentration in the pellets 1300 a by (a) atleast 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19,and/or 20 absolute wt %, (b) less than 30, 25, 20, 19, 18, 17, 16, 15,14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, and/or 1 absolute wt %,and/or (c) a value between any two such values (e.g., between 1 and 30absolute wt %, between 5 and 25 absolute wt %, between 10 and 20absolute wt %, and/or about 15 absolute wt %).

As used herein, ‘absolute wt %” means percentage of the overall fuelcontent, not a percentage deviation from another value. As a result, afuel pellet 1300 a with a 60 wt % thorium fuel concentration has 15absolute wt % less concentration than a fuel pellet 1300 b with a 75 wt% thorium fuel concentration.

According to various non-limiting embodiments, the composition (e.g.,²³⁵U enrichment level, thorium/uranium concentration, poisonconcentration) of all fuel pellets 1300 a in the central pin 1200 aand/or the first ring pins 1200 b may be the same. According to variousnon-limiting embodiments, the composition of all fuel pellets 1300 b inthe second ring pins 1200 c may be the same. According to variousnon-limiting embodiments, the composition of all fuel pellets 1300 c inthe outer ring pins 1200 d may be the same. According to variousalternative embodiments, different pins within a given ring may utilizepellets with different compositions. According to various alternativeembodiments, the fuel pellets within a given pin may have differentcompositions than each other.

According to various non-limiting embodiments, the fuel is utilized inan open fuel cycle where thorium is converted to U233 which is burnedin-situ. According to various non-limiting embodiments, the fuel isburned to levels that are not achievable with current CANDU/PHWR fuels.According to various non-limiting embodiments, residual amounts offissile material is not significant for use in non power reactorapplications. The fuel is characterized by inherent safety features suchas strongly negative fuel temperature reactivity coefficient and reducedcoolant void reactivity.

Alternative Reactors

The above described embodiments involve fuel 100, 200, 300 for a 220 MWePHWR or fuel 1100, 1200, 1300 for a CANDU 600 MWe reactor. However,various embodiments can be used on other types of PHWRs withoutdeviating from the scope of the invention. For example, various of theabove-described embodiments can be applied to other Indian PHWRs (e.g.,Indian 540 MWe PHWR, Indian 700 MWe PHWR) or other CANDU reactors (e.g.,300 or 900 MWe), and may use fuel bundles with greater or fewer pinsthan the 19-pin and 37-pin embodiments in the above-describedembodiments.

Unless otherwise explicitly stated, the compositions (e.g., in terms ofwt %, fuel, burnable poison, etc.) herein for pellets, pins, and fuelbundles refer to the composition for a fresh, non-irradiated pellet,pin, or fuel bundle before being used in the reactor, rather than thecomposition after irradiation within the reactor. A fresh pellet maycomprise recycled uranium, depleted uranium, natural uranium, and/oruranium from any other source, including sources of previouslyirradiated uranium. Similarly, a fresh pellet may comprise thorium fromany source, including sources that have been previously irradiated.

The foregoing illustrated embodiments are provided to illustrate thestructural and functional principles of various embodiments and are notintended to be limiting. To the contrary, the principles of the presentinvention are intended to encompass any and all changes, alterationsand/or substitutions thereof (e.g., any alterations within the spiritand scope of the following claims).

1. A fresh fuel pellet configured to be used in a pressurized heavywater reactor, the fuel pellet comprising thorium and uranium fuel,wherein a fuel composition of the fuel pellet is between 55 and 90 wt %thorium, wherein the fuel composition is between 10 and 45 wt % uranium,and wherein a ²³⁵U enrichment of the uranium is between 10.5% and 20%.2. The fuel pellet of claim 1, wherein the pellet is annular in shapewith a through-hole.
 3. (canceled)
 4. The fuel pellet of claim 1,wherein the fuel composition is between 70 and 90 wt % thorium.
 5. Thefuel pellet of claim 1, wherein the ²³⁵U enrichment of the uranium isbetween 15% and 19%.
 6. A fuel pin configured to be used in apressurized heavy water reactor, the fuel pin comprising: a sealed tube,and a plurality of fuel pellets according to claim 1, wherein theplurality of fuel pellets are disposed inside the sealed tube.
 7. Thefuel pin of claim 6, wherein a fuel composition of the fuel pin isbetween 70 and 85 wt % thorium.
 8. The fuel pin of claim 6, wherein eachof the plurality of fuel pellets comprises burnable poison.
 9. A fuelbundle configured to be used in a pressurized heavy water reactor, thefuel bundle comprising a plurality of fuel pins according to claim 6,wherein a fuel composition of at least one of the plurality of fuel pinsdiffers from a fuel composition in at least one other of the pluralityof fuel pins.
 10. The fuel bundle of claim 9, wherein: the fuel bundleis shaped and configured to be used in a 220 MWe PHWR; the plurality offuel pins has exactly 19 fuel pins comprising one central fuel pin, sixintermediate fuel pins disposed radially outwardly from the one centralfuel pin, and twelve outer fuel pins disposed radially outwardly fromthe six intermediate fuel pins; and a thorium weight percentage of thefuel composition of the central fuel pin is lower than a thorium weightpercentage of the fuel composition of each of the twelve outer fuelpins.
 11. The fuel bundle of claim 10, wherein: the fuel composition ofeach of the central fuel pin and six intermediate fuel pins has athorium content of between 55 and 75 wt %; the fuel composition of eachof the twelve outer fuel pins has a thorium content of between 65 and 90wt %; and each of the twelve outer fuel pins has a higher wt % thoriumcontent than each of the central and six intermediate fuel pins.
 12. Thefuel bundle of claim 10, wherein: the fuel composition of the centralfuel pin has a lower wt % thorium content than in each of the sixintermediate fuel pins; and the fuel composition of each of the sixintermediate fuel pins has a lower wt % thorium content than in each ofthe twelve outer fuel pins.
 13. The fuel bundle of claim 12, wherein:the fuel composition of the central fuel pin has a thorium content ofbetween 55 and 70 wt %; the fuel composition of each of the sixintermediate fuel pins has a thorium content of between 60 and 80 wt %;and the fuel composition of each of the twelve outer fuel pins has athorium content of between 65 and 90 wt %.
 14. The fuel bundle of claim10, further comprising burnable poison disposed in the sealed tube ofeach of the six intermediate fuel pins.
 15. The fuel bundle of claim 14,wherein burnable poison is not disposed in the sealed tube of any of theouter fuel pins.
 16. The fuel bundle of claim 10, further comprisingburnable poison disposed in the sealed tube of each of the central andsix intermediate fuel pins, wherein burnable poison is not disposed inthe sealed tube of any of the outer fuel pins.
 17. (canceled)
 18. Thefuel bundle of claim 10, further comprising burnable poison disposed inthe sealed tube of each of the twelve outer fuel pins.
 19. The fuelbundle of claim 18, wherein burnable poison is not disposed in thesealed tube of the central fuel pin or any of the intermediate fuelpins.
 20. The fuel bundle of claim 10, wherein burnable poison is notdisposed in the sealed tube of any of the fuel pins of the fuel bundle.21. (canceled)
 22. The fuel bundle of claim 10, wherein a ²³⁵Uenrichment of the uranium of each of pellets of each of the central,intermediate, and outer pins is at least 15%.
 23. The fuel bundle ofclaim 21, wherein a ²³⁵U enrichment of the uranium of each of thepellets of each of the twelve outer pins is lower than a ²³⁵U enrichmentof the uranium of each of the pellets of each of the central and sixintermediate pins.
 24. The fuel bundle of claim 9, wherein: the fuelbundle is shaped and configured to be used in a CANDU PHWR; and theplurality of fuel pins comprise exactly 37 fuel pins including: onecentral fuel pin, six first ring fuel pins disposed radially outwardlyfrom the one central fuel pin, twelve second ring fuel pins disposedradially outwardly from the six first ring fuel pins, and eighteen outerfuel ring pins disposed radially outwardly from the twelve second ringfuel pins.
 25. The fuel bundle of claim 24, wherein a thorium weightpercentage of the fuel composition of the central fuel pin is lower thana thorium weight percentage of the fuel composition of the second ringpins and of the outer ring pins.
 26. The fuel bundle of claim 24,wherein: a thorium weight percentage of the fuel composition of thecentral fuel pin and of the first ring fuel pins is between 50 and 70 wt%; a thorium weight percentage of the fuel composition of the secondring fuel pins is between 60 and 90 wt %; and a thorium weightpercentage of the fuel composition of the outer ring fuel pins isbetween 75 and 99 wt %.
 27. The fuel bundle of claim 24, wherein a ²³⁵Uenrichment of the uranium of the central fuel pin and first ring fuelpins is higher than a ²³⁵U enrichment of the uranium of the second ringpins or outer ring pins.
 28. The fuel bundle of claim 27, wherein a ²³⁵Uenrichment of the uranium of the second ring fuel pins is higher than a²³⁵U enrichment of the uranium of the outer ring pins.
 29. The fuelbundle of claim 24, wherein the central pin, first ring pins, and secondring pins each comprise burnable poison.
 30. (canceled)
 31. The fuelbundle of claim 29, wherein the outer ring pins do not include burnablepoison.
 32. A pressurized heavy water reactor comprising: a reactorvessel; and a core disposed in the reactor vessel, the core comprising aplurality of fuel bundles according to claim 9, wherein the plurality offuel bundles comprise a first type of fuel bundle and a second type offuel bundle, wherein the first type of fuel bundle comprises burnablepoison, and wherein the second type of fuel bundle does not compriseburnable poison.
 33. The pressurized heavy water reactor of claim 32,wherein the first and second types of fuel bundles are identical to eachother except for the inclusion of burnable poison in the second type offuel bundle.
 34. A fuel pellet configured to be used in a pressurizedheavy water reactor, the fuel pellet comprising thorium and uraniumfuel, wherein a fuel composition of the fuel pellet is between 55 and 90wt % thorium, wherein the fuel composition is between 10 and 45 wt %uranium, wherein a ²³⁵U enrichment of the uranium is between 5% and 20%,and wherein the pellet is annular in shape with a through-hole.